1. Field of the Invention
The present invention relates to improvements to communications systems. More particularly, the present invention relates to wireless discrete tone communications systems.
2. Description of the Related Art
Wireless communications systems, such as cellular and personal communications systems, operate over limited spectral bandwidths and must make highly efficient use of the scarce bandwidth resource for providing good service to a large population of users. Wireless systems consist of cells, each of which include a base station and remote units. The Personal Wireless Access Network described in the Alamouti, et al patent application cited above, is an example of a successsful technology for wireless service.
The remote units are powered primarily from AC power sources and include a battery for providing battery backup power when AC power fails. To conserve battery power, an RU has a sleep mode of operation with periodic power-up modes for checking whether any calls are attempting to be connected to the RU. When an RU is in a sleep mode, it expedient that the system operate in such a way so that appropriate actions are taken for completing a call to a sleep mode RU.
One approach for ensuring that calls are completed to a remote unit operating in a sleep mode is to maintain a database at a central location that stores the current operating mode of each remote in the system. When a remote unit enters a sleep mode of operation, the remote unit reports the change of operational status to the database. Similarly, the remote unit reports a change of status back to a standby operating mode. This approach has a drawback when a number of remote units recorded in the database experience frequent power outages. In such a situation, recording, managing and synchronizing power outage information in the database is particularly cumbersome when the database is large, perhaps holding status information for 3 to 4 thousand remote units. This drawback is further compounded when the database is duplicated multiple times throughout the system. When several thousand subscribers experience a power outage and AC power is restored before the database has completed recording the power outage, a database approach becomes unwieldy. Another complicated situation is when multiple remote units lose power at the same time. The affected remote units cannot all access the channel simultaneously for communicating their status to the database. A collision avoidance scheme must be implemented that spans a period of time and that is open for the possibility of power being restored before the database has been completely revised.
This approach has another drawback in that a remote unit entering the sleep mode consumes system bandwidth in notifying the database. FIG. 4 shows an exemplary flow of internal messaging that occurs between various layers of a remote unit when loss of AC power is detected and a database is notified of the operational status change. Time is shown along the vertical axes of FIG. 4, with advancing time being indicated toward the bottom of FIG. 4. In FIG. 4, four layers of the remote unit operating system are shown: Health; OAM&P (Operations, Administration, Maintenance & Provisioning), MAC (Media Access Control) and physical. Only MAC layer of the base station is shown. At 40, AC power failure is detected by the Health layer. At 41, an EVENT message is sent from the Health layer to the OAM&P layer indicating that AC power has failed. The OAM&P layer sends an ACTION message to the MAC layer at 42. The MAC layer responds at 43 by sending an ACTION.sub.-- RSP message to the OAM&P layer indicating that base station notification is pending. At 44, the MAC layer waits a random length period of time before sending an unsolicited CAC message at 45 to the MAC layer of the base station indicating the need for the remote unit to enter the sleep mode. At 46, the MAC layer of the base station sends an acknowledgment message to the MAC layer of the remote unit acknowledging receipt of the unsolicited CAC message. In response, the MAC layer of the remote unit sends an EVENT message at 47 to the OAM&P layer that the notification is done. The OAM&P layer first sends an EVENT message to the MAC layer indicating that the sleep mode has been entered at 48, and then sends a message at 49 to the physical layer to power down.
What is needed is a way for a PWAN system to be aware that a remote unit is operating in a sleep mode so that appropriate actions can be taken by the system so that calls can be completed to a remote unit operating in a sleep mode.